Opal glasses are produced by forming, or maintaining, in a glass a second phase that has a sufficiently different refractive index from that of the glass to effectively scatter transmitted light and thereby create an opaque appearance. In the absence of glass colorants or pigments, an opal glass generally appears white, and the density of the opal depends, among other things, on the relative difference in refractive indices and the amount of opal phase present in the glass.
Opal glasses are widely used in both culinary and tableware. The mechanical strength of such ware may be enhanced by thermal tempering, thus permitting a thinner wall, and hence lighter weight, in the ware. As wall thickness of an article is decreased, however, the need for a high opal density becomes more critical in order to avoid a translucent effect in the thin wall.
It is of course possible to use opal glassware as molded, that is, in the plain white state with no decorative coating applied. However, most homemakers demand a decorative appearance on their cookware and tableware, and a wide variety of colors and colored patterns has been developed to satisfy this need.
Decorative enamels have been employed extensively in the past to impart color and/or gloss to opal glass articles. Such enamels customarily are composed of a clear glass base, referred to as the flux, and a mill-type addition, referred to as the pigment, the latter supplying the coloring effect in the enamel. The enamel, consisting of intimately mixed flux and pigment, is applied to the surface of the glass in frit form and fired to form an adherent, continuous coating on the glass.
The firing temperature must be sufficiently high to permit the enamel flux to completely dissolve and/or suspend the pigment in a smooth homogeneous coating, while not becoming so soft as to separate or flow away from the pigment. However, the firing temperature of the enamel must not be so high as to cause thermal deformation of the opal glass substrate. Typically, flat tableware, such as dinner plates and saucers, must be fired at lower temperatures, than items such as cups, casseroles, or serving bowls. Hence, lower firing temperatures, and consequently lower softening point enamel fluxes, are required to prevent sagging of these flat tableware bodies. Also, the enamel will normally have a coefficient of thermal expansion somewhat below that of the opal glass in order to inhibit crazing or spalling and to place the enamel coating in compression with respect to the glass after the firing step.
It has become customary to include substantial amounts of lead oxide and also, in some cases, cadmium oxide in an enamel flux composition to provide a high index of refraction and low melting point. Also, cadmium compounds are frequently used as yellow and red enamel pigments, and, therefore, CdO must be present in the enamel flux to allow sufficient dissolution of the cadmium-bearing pigment in order to develop the desired color. It is well known, however, that lead and cadmium are extremely toxic metals, and that enamels containing these metals must be highly resistant to chemical attack by such varied chemicals as acids, alkalis, and sulfides in order to prevent appreciable release of these toxic metal elements.
In recognition of the potential danger from excessive toxic metal release, the Food and Drug Administration (FDA), in its Compliance Guidance Manual issued June 13, 1974, has established maximum limits which lead and/or cadmium release from an enameled surface must not exceed. In the prescribed FDA test, an enamelled surface is exposed for 24 hours to 4% acetic acid at room temperature (22.+-.2.degree. C.). A sample of the acid solution is then tested for absorbance in an atomic absorption spectroscope and the observed value converted to a metal concentration value on a standard curve, the metal being reported in parts per million (ppm). The reported value is based on the inside volume of a hollow article having an enamelled or decorated inner surface and filled to a specified level with acetic acid for the test. A similar test has been devised for use on exterior surfaces of a vessel or dish as well.
In order to comply with FDA requirements, lead release from a food contacting surface, that is, the inside of a dish which actually contacts food during preparation, service or storage, must not exceed 7.0 parts per million (ppm) and cadmium release correspondingly must not exceed 0.5 ppm. It is of course desirable to employ enamels having release values well below these limits at any time during the expected life of an enamelled article. To this end, arbitrary lifetime standards of less than 1.3 ppm lead and less than 0.3 ppm cadmium have been established by the applicant.
While the FDA standards are based on acid reaction on a freshly produced article, it is well known that alkaline solutions may be even more detrimental to a vitreous surface. Accordingly, a test has been devised in which weighed and measured samples of enameled glass are immersed in a 0.3% by weight aqueous solution of an alkaline detergent marketed by Economics Laboratories, St. Paul, Minn., under the mark Super Soilax. The solution is maintained at 95.degree. C. for 24 hours, after which the samples are removed, rinsed, dried, and weight loss determined. The loss may be based on the enamel, per se, or may be reported as lead and/or cadmium release values for comparison with FDA standards. The time of 24 hours represents an accelerated equivalent to the anticipated exposure of a dish to such conditions during its expected lifetime.
It has been observed above that an enamel is applied as a milled mixture of a flux or vitreous portion and a pigment additive. The pigment is normally a minor part of the enamel, added in amounts less than 20%, and frequently less than 10%, to impart opacity and/or color to the enamel. The physical properties and sealing characteristics of an enamel are determined primarily by the flux or vitreous base component, and therefore it is this material upon which research is primarily concentrated for improvement and/or modification of characteristics other than color. However, metal release is measured on the fired enamel, that is, the flux plus pigment.